1. Field of the Invention
The present invention relates to a method for manufacturing a soft magnetic steel sheet for use as the iron core of transformers, motors and the like. More specifically, the present invention relates to a method for manufacturing a silicon steel sheet showing a low iron or core loss and a high magnetic flux density, in which the silicon steel sheet is manufactured by a direct casting method, and then a rolling and a heat treatment are carried out.
2. Description of the Prior Art
Generally, the non-oriented silicon steel sheet is manufactured in the following manner. That is, an ingot is hot-rolled and cold-rolled, and then, an annealing is carried out to relieve the stress. In this silicon steel sheet thus manufactured, it is demanded that the soft magnetic material properties (iron loss) should be superior. Therefore, all the process steps of the conventional method concentrate on minimizing the iron or core loss.
The known methods for reducing iron or core loss of silicon steel sheet include a method of facilitating the movements of the magnetic domains, a method of increasing the resistivity, and a method of forming an advantageous texture and decreasing a disadvantageous texture. Specifically, in order to facilitate the movements of the magnetic domains by improving the purity, impurities such as oxygen, carbon, nitrogen and titanium are removed. In the method of increasing the resistivity, the contents of elements such as silicon, aluminum and manganese are increased. In this context, care should be exercised in such a manner that the ductility of the silicon steel sheet should not be aggravated. Further, even in materials having the same composition, the magnetic properties are varied in accordance with the major grain orientation, and therefore, efforts have been made to improve the magnetic properties by resorting to the texture. For example, it is known that if tin and antimony are added, the crystallographic plane (200) which is favorable to the magnetic properties is increased.
However, the conventional method for manufacturing the silicon steel sheet by using an ingot reached a limit in purifying the material, in adding alloying elements, and in improving the texture.
Meanwhile, recently, a direct casting method which is quite different from the conventional methods has been developed. In the silicon steel sheet manufactured by the direct casting method, a high density (200) texture is observed even at a thickness of more than 200 .mu.m, as well as showing superior magnetic properties.
The present inventor also proposed a method for manufacturing a high (200) plane density silicon steel sheet by employing a single rolling process in Korean Patent Application No. 95-48472 (dated Dec. 11, 1995). However, the silicon steel sheet which is manufactured by the direct casting method does not show a good flatness, because the sheet is directly cast in the molten pool. Further, its thickness is irregular, and therefore, a large stacking is difficult. Therefore, it could not be put to the practical use. Actually in the case of Fe-2.5%Si in a direct casting method, the surface roughness of the sheet is about 5 .mu.m as shown in FIG. 1, and therefore, it cannot be used as it is. Further, in the case where the content of Si is high, the silicon steel sheet which is manufactured by the direct casting is brittle, and therefore it cannot be put to the practical use, in spite of its superior magnetic properties.